The long-term objective of this research is to elucidate mechanisms which regulate the physiology of excitable cells. In the studies proposed, the three microelectrode voltage clamp will be used to probe electrical excitability in mammalian skeletal muscle. In initial studies of this tissue, a calcium current and slow outward current have been identified. The kinetics and voltage-dependence of the calcium current will be measured in Cs-loaded fibers and three hypotheses for the inactivation of the current will be tested. Also to be tested is the hypothesis that activation of the current is sufficiently rapid under physiological conditions to promote appreciable calcium entry during normal fiber electrical activity. By using ion substitution and the addition of Ca channel blockers, the absence or presence of a calcium-activated potassium current will be established. Circumstantial evidence suggests that calcium and calcium-activated potassium currents become much more prominent following denervation. This hypothesis will be tested directly by measuring these slow ionic currents in denervated muscle. The action of beta-adrenergic amines on slow ionic currents will be established, both in normal and denervated muscle. Voltage-dependent charge movement, which is thought to regulate calcium release from the sarcoplasmic reticulum, will be studied and its steady state distribution and kinetics will be characterized. As part of this effort it will be determined whether a pharmacologically-labile component of charge which has been identified in frog muscle is also present in rat muscle. Charge movement will be measured before and after treatment with catecholamine to test the hypothesis that the hormone's potentiation of the twitch occurs at the level of charge movement. Measuring charge movement over a range of temperature will allow more accurate inferences about its kinetic behavior and its distribution within the transverse tubular system.